1. Field of the Invention
The present invention relates to a brushless motor that has a skew structure. More particularly, the invention relates to a brushless motor having a step-skew structure by means of segment magnets.
2. Related Art Statement
Hitherto, harmonic components have been used in brushless motors for use in electric power steering (EPS) apparatuses, changing the waveform of the line induced voltage between any adjacent lines to a trapezoidal waveform to increase the rate of utilization of motor induced voltage so that the motors may be small and generate a large output as demanded. Generally, the voltage induced in a brushless motor can contain a prominent fifth harmonic component and a prominent seventh harmonic component if the motor has an integral multiple of 2 poles and 3 slots (this will be hereinafter called “2P3S×n”). If the induced voltage contains a fifth harmonic component, in particular, it will have a trapezoidal waveform. Accordingly, in a brushless motor for use in EPS apparatuses, the 2P3S×n structure is used and a fifth harmonic component is contained in the induced voltage, in order to increase the output of the brushless motor, while reducing torque ripple.
Hitherto known, as a technique for containing a fifth harmonic component in the induced voltage to change the waveform of the induced voltage to a trapezoidal one, is the skew structure in which the rotor poles or the like are inclined to the axis of the rotor. In most brushless motors of skew structure, ring magnets are used as pole magnets. In any motor having ring magnets, the magnets themselves are skew-magnetized in order to reduce cogging torque and torque ripple.
In recent years, the segment magnets that can be magnetized at high flux density have been increasingly used as rotor magnet in brushless motors designed for use in EPS apparatuses, thereby to meet the demand that the motors should be smaller and should yet achieve a large output. However, the segment magnets may have bands not magnetized as shown in FIG. 10 if skew-magnetized. Inevitably, the magnet material is wasted in any motor using segment magnets. Consequently, the motor involves the problem of cost-performance.
In any motor that has segment magnets, the magnetized segment magnets are therefore stacked one on another, thus achieving a so-called step skew in order to achieve a skew structure. In the motor with the step-skew structure, the segment magnets are arranged in even-number stages (usually, two stages), in the axial direction in order to offset the cogging waves at the respective steps to reduce the cogging torque. Patent Document 1 discloses a rotating electrical machine in which magnets are arranged in two stages. In the rotating electrical machine disclosed in the patent, the magnets of each stage are arranged in the circumferential direction, each magnet being shifted at a specific angle from the next. The poles of the rotor are thereby displaced stepwise in the axial direction, whereby a two-staged, step-skew structure is constructed.
In the step-skew structure, however, assembled state, physical properties, process precision, and the like actually have a variation from the design specification. Inevitably, the cogging torque cannot be reduced so much in the two-staged, step-skew structure. Also, if the skew angle is set to a value close to theoretical mechanical skew angle=360°/(the common least multiple of the poles and slots), the higher harmonic components of the induced voltage will abruptly decrease. In view of this, the skew angle is usually set to a value smaller than the theoretical mechanical skew angle. However, such a small skew angle can hardly serve to reduce the cogging torque resulting from the two-stage step-skew structure.
Also, in order to reduce the cogging torque, a technique is widely used in which auxiliary grooves are made in the distal end of each stator core teeth, thereby providing pseudo slots. If the fundamental wave contains harmonic components resulting from the auxiliary slots, however, the cogging torque will increase in the two-stage step-skew structure. Any motor for use in EPS apparatuses, in particular, must be so designed, not only to generate a large output, but also to reduce the cogging torque and torque ripple. Therefore, it is important for such a motor to generate a large output and, at the same time, reduce the cogging torque and torque ripple.